The present invention relates to a global positioning system/inertial measurement unit process and system thereof, and more particularly to a fully-coupled kinematic global positioning system/inertial measurement unit process and system thereof to improve the navigation accuracy of a GPS guided vehicle on land, air, and space.
To meet the future applications, it is needed to develop a reliable, accurate, miniaturized, low cost, kinematic global positioning system/inertial measurement unit integrated navigation system which is capable of operating in a high dynamic flight environment against a mixture of multi-type global positioning system (GPS) signal loss or deterioration, and improving navigation accuracy of GPS guided vehicle.
A major way of reducing cost of a navigation system is to use cheaper sensors and components that make the integrated navigation, and guidance and control system designs all the more challenging. Traditionally, guidance and navigation systems used for guided vehicle are mainly inertial navigation systems (INS) which is composed of an inertial measurement unit (IMU) and a processor. An important advantage of INS guidance is independence from external support. Unlike other types of guidance, INS devices can not be jammed or fooled by deceptive countermeasures. Unfortunately, INS guidance cannot provide high accuracy at long ranges. Inertial sensors are subject to errors that tend to accumulate over timexe2x80x94the longer the flight time, the greater the inaccuracy. The cost of developing and manufacturing a gyroscope increases as its level of accuracy improves. High-quality gyroscopes are difficult to manufacture, and only a relatively small number of companies around the world are capable of producing them. In part, this reflects the limited market for gyroscopes suitable for use in a highly accurate INS. Therefore, the inherent inaccuracy of the INS means that it cannot be the sole guidance system for a highly accurate tactical missile. Additional inputs are needed to correct for INS errors.
More recent developments in satellite navigation techniques are making possible the precise navigation at low cost. Efforts are now under way to develop integrated GPS/IMU navigation and guidance systems, suitable for use in high-jamming and high-dynamic flight environments. One implication of integrated GPS/IMU packages is that lower-cost, more easily manufactured IMU sensors can be used. This can result in significant savings.
Therefore, the technology trends for inertial sensors, GPS accuracies, and integrated GPS/IMU systems, including considerations of jamming and high dynamic, will lead to the one meter accuracy. The technical challenges come from the improvement of inertial and GPS sensor performance and the optimal integration of these sensors in the software and hardware designs. For inertial sensors, trend-setting sensor technologies are fiber-optic gyros, silicon micromechanical gyros, resonating beam accelerometers, and silicon micromechanical accelerometers. The utilization of these techniques is resulting in low-cost, high reliability, small size, and light weight for inertial sensors and for the systems into which they are integrated.
For the GPS accuracies, the current 16-meter (SEP, spherical error probable) specified accuracy, or 8 to 10-meter (CEP, circular error probable) observed accuracy of the GPS PPS (Precise Positioning Service) provides impressive navigation performance especially when multiple GPS measurements are combined into a robust centralized Kalman filter to update an INS. The filter provides an opportunity dynamically to calibrate the GPS errors, as well as, the inertial errors, and when properly implemented, CEPs far better than 8 meters can be obtained. For example, for precision guidance and automated aircraft landings, the requirement for accuracy of the integrated navigation systems is less than 3 meters or even better.
The trend towards improvement of accuracies of the integrated navigation systems is to utilize kinematic GPS and develop advanced fully-coupled kinematic GPS/IMU integrated systems in which both GPS receiver code and carrier tracking loops are aided with the inertial sensor information. Therefore, the measurement accuracy and anti-jamming capability of the GPS receiver can be dramatically enhanced and increased. Rapid carrier integer cycle ambiguity search and resolution, cycle slip detection and isolation procedures can also be completed within a few seconds through use of the inertial aiding information. In addition, GPS specified and observed current accuracies can be improved due to various stages of the wide area GPS enhancements.
As a result, the design and development of kinematic GPS/IMU integrated navigation systems is extremely challenging. Specifically, the hardware sensors and software algorithms constituting the system should satisfy the following requirements:
1. Inertial Sensors
Major changes are currently underway in technologies associated with inertial sensors used for stabilization, control, and navigation. These changes are enabling the proliferation of inertial sensors into a wide variety of new military and commercial applications. Main Challenges for design and fabrication of inertial sensors are low cost, high reliability, accuracy required by mission, small size, and lightweight.
(1) Fiber-Optic Gyros (FOG)
An economical replacement for the ring laser gyro (RLG) providing the same level of gyro bias performance.
(2) Silicon Micromechnical Gyros
Continuous reduction in the gyro drift rate for more demanding applications.
(3) Resonating Beam Accelerometers
(4) Silicon Micromechanical Accelerometers
2. GPS receiver
As regards GPS sensor size, the current GPS receiving card (OEM, original equipment manufacture) is less than the size of a cigarette box. Technical trades for design of GPS receivers for GPS guided vehicle will focus on enhancement of high anti-jamming and high dynamic performance, and decrease of GPS measurement noise, including multipath effects.
(1) Trade-off between tracking loop bandwidth and high anti-interference of GPS receiver.
(2) Short time-to-first-fix (TTFF) and signal reacquisition time.
(3) Direct rapid P-code tracking and capturing.
(4) Inertial aiding code and carrier tracking loops.
(5) Receiver hardware/software digital signal processing.
(6) Anti-multipath antenna design.
3. Integrated System Algorithms
In future GPS/IMU integrated navigation systems, the fully-coupled integration requires that the GPS measurements and inertial sensor information are directly fused into a centralized navigation Kalman filter, and outputs of the filter can also aid the receiver tracking loops to improve the anti-jamming capability of the GPS receiver. Therefore, the technical challenges will be the following:
(1) System reconfiguration based on multiple sensors.
(2) Multi-mode robust navigation Kalman filter.
(3) Sensor failure detection and isolation.
(4) Inertial aiding on-the-fly phase ambiguity resolution and cycle slip detection.
(5) Rapid transfer alignment.
The current technical innovation will contribute significantly to the prospects for high dynamic vehicle proliferation. Historically, the most significant obstacles to the design and development of high dynamic guided vehicle have been the cost and complexity of vehicle guidance systems.
The main objective of the present invention is to provide a fully-coupled positioning process and system thereof, which is an innovative fully-coupled GPS/INS algorithm for enhancing the performance of GPS/INS integration navigation system in heavy jamming and high dynamic environments, that utilizes the GPS carrier phase information to determine highly Time Space Position Velocity Information (TSPVI).
Another objective of the present invention is to provide a fully-coupled positioning process and system thereof, in which an advanced fully-coupled GPS/INS integrated system architecture is developed, which makes possible the implementation of mutual error compensation and aiding between GPS and IMU from the view of hardware and software implementation. The architecture provides the most cost-efficient approach for the implementation of hardware/software systems and the aiding of GPS with INS data.
Another objective of the present invention is to provide a fully-coupled positioning process and system thereof, in which a novel V-A (velocity-acceleration) aiding GPS signal tracking loop algorithms including code tracking loop algorithm and carrier tracking loop algorithm have been completed. Under the new architecture of GPS/INS integration, both the GPS receiver""s code and carrier tracking loop can be aided by INS data at a high rate of data, which dramatically increase the measurement accuracy, dynamic tracking capability, and anti-jamming capability of GPS receiver.
Another objective of the present invention is to provide a fully-coupled positioning process and system thereof, in which an innovative IMU aiding widelane carrier phase ambiguity resolution on-the-fly algorithm is developed, which can provide highly and precise carrier phase measurements for the integrated navigation Kalman filter. The approach significantly reduces the time spent for an ambiguity search procedure and increases the resolution of ambiguity.
Another objective of the present invention is to provide a fully coupled positioning process and system thereof, in which a robust integrated navigation Kalman filter is implemented in real time. The filter more effectively utilizes all available measurements and a prior information, including GPS pseudorange (PR), delta range (DR), carrier phases measurements, inertial measurement information, to determine and correct for system errors in a fully-coupled fashion.
Another objective of present invention is to provide a fully-coupled positioning process and system thereof, in which a novel algorithm for rapid transfer alignment and calibration for both aircraft INS and munition INS attitude is studied. This algorithm is used to remove initial position, velocity and attitude errors of tactical munitions.
Another objective of present invention is to provide a fully-coupled positioning process and system thereof, in which a real-time kinematic GPS/IMU integrated navigation software system is implemented, which also provide a tool for development of different levels of GPS/IMU integrated navigation systems adaptable to wide usage applications.
Another objective of present invention is to provide a fully-coupled positioning process and system thereof, in which a navigation computer system is designed, that directly points to a broad class of military/civilian/government applications including strike weapons, unmanned airborne vehicle and avionics platforms.
Another objective of present invention is to provide a fully-coupled positioning process and system thereof, which not only provides a solid basis and powerful tools for the improvement of accuracy of the navigation systems used for the guided vehicle, but also creates a new trend and open new directions for further investigation of challenging problems faced by designs of advanced navigation systems for high dynamic vehicle.
Accordingly, in order to achieve the above objectives, the following innovative technical features have to bring to our investigation.
1. Optimal Integrated Mode: Position and velocity (P-V) integrated method, pseudorange and delta range (xcfx81/xcex94xcexd+xcex94xcex8) integrated mode without IMU aiding the GPS tracking loops, kinematic integration mode xcfx81+xcfx86/xcex94xcexd+xcex94xcex8 without IMU aiding the GPS tracking loops, (xcfx81/xcex94xcexd+xcex94xcex8) integrated mode with IMU aiding the GPS tracking loops and xcfx81+xcfx86/xcex94xcexd+xcex94xcex8 with IMU aiding the GPS tracking loops. The comparison of the existing different integration approaches led to the option of the optimal integration architecture for the fully-coupled kinematic GPS/IMU integrated navigation system.
2. Innovative technique for inertial aiding of the GPS tracking loops: One of the technology trends towards GPS/IMU integrated systems is to develop a fully-coupled kinematic GPS/IMU integrated system, where the GPS receiving set""s code and carrier tracking loops are aided with inertial sensor information. We have tried to make use of the most updated results in our IMU aiding GPS tracking loops algorithm to improve GPS measurement accuracy and anti-jamming capability in a tactical dynamic environment.
3. Novel inertial information aiding phase ambiguity resolution technique: High dynamic kinematic GPS navigation is limited by the ability to resolve the carrier integer cycle ambiguity in a timely manner. An IMU aiding widelane ambiguity resolution technique is utilized to significantly reduce the time for the ambiguity search procedure and to obtain highly reliable ambiguity solutions in a high dynamic environment. The method can also be used for the resolution of carrier integer cycle ambiguity in the single-frequency kinematic GPS measurements.
4. Robust centralized integrated Kalman filter: A complete approach to reliable, robust, and adaptable Kalman filter is developed which can operate in more than one dynamic environment to predict the actual system performance. This type of filter configuration has many advantages over the usual Kalman filter such as a larger region of convergence, smoother transitions between over-determined solutions and more conservative modeling when certain states are frozen, such as during clock or altitude hold. In addition, the centralized filter approach to kinematic GPS/IMU integrated algorithm avoids filter instability problems as the filter-driving-filter configuration meets.
5. Rapid transfer alignment.
Furthermore, a highly challenging research topic in the development of a new generation integrated guidance navigation systems using low-cost IMU sensors is to develop a fully-coupled kinematic GPS/IMU navigation system adaptable to a high dynamic flight environment. The term fully-coupled means that the IMU and GPS directly complement each other. The IMU-derived velocity and acceleration (V-A) information can be used to aid a GPS receiver""s code and carrier phase locked-loops for tracking the Doppler-drifted satellite signals. And vice-versa, the long-term stability and accuracy of the GPS position and velocity information can be utilized to compensate and calibrate the bias and drift errors of the IMU sensor. Several possible levels of hardware and software integration methods have currently been presented for the various purposes of GPS/IMU integration. According to the traditional category method, the architecture of the integration system is classified into two types: loosely-coupled and tightly-coupled systems. Generally speaking, the loosely-coupled system has an unambiguous definition and requirements for GPS and INS. For example, the loosely-coupled system needs the independent navigation solutions from GPS and INS systems, respectively. But, the tightly-coupled system is easily confused from the view of availability of the GPS and IMU measurement information. For example, fusion of either pseudorange or carrier phase measurements into the integrated Kalman filter leads to different requirements for the GPS receiver and different integrated algorithms for data processing. The information flow between the IMU and the GPS receiver depends on levels of the GPS/IMU integration. Therefore, we classify the integrated GPS/IMU system into 5 types of integration modes from the view of information fusion.
A. GPS/INS P-V integration mode: Traditionally, it is also called the loosely-coupled mechanization. In the integrated system, the GPS and INS are considered as independent navigation systems, as shown in FIG. 1.
The integrated navigation solutions are provided by a separately integrated navigation Kalman filter, which directly utilizes the navigation solutions (position and velocity or time and attitude) derived by the GPS and INS navigation systems, respectively. The GPS P-V solution can correct the INS solution errors periodically. Theoretically, the IMU-derived V-A solution can aid the GPS receiver tracking loops if the GPS receiver hardware/software systems are properly designed. But, it is practically difficult because the loosely-coupled mechanization has a cascaded filter performance with which the integrated navigation Kalman filter can not provide the GPS tracking loops with a high rate data input. The GPS/INS P-V integrated navigation systems can be found in military GPS applications in the past decades. One disadvantage of the GPS/INS P-V integration system is that cascaded filter performance can be degraded by correlations in the data. Care must be taken to ensure that the time-correlated outputs of the GPS filter do not cause stability problems in the integrated navigation filter. Another disadvantage of the loosely coupled architecture is that the GPS filter can experience large errors in the presence of high receiver dynamics; this may necessitate aiding from the integrated navigation filter, which can worsen the correlation problem. But, an obvious advantage of the loosely coupled technique is that it allows maximum use of off-the-self hardware and software that can be easily assembled into a cascaded system without major new development.
B. GPS/IMU xcfx81/xcex94xcexd+xcex94xcex8 Integration Mode without Aiding of GPS Tracking Loops: In the GPS/IMU xcfx81/xcex94xcexd+xcex94xcex8 integration mode, the integrated navigation Kalman filter directly fuses and processes the raw measurement data from the GPS and IMU sensors, respectively, such as GPS pseudorange (PR) and delta-range (DR) measurements, and inertial indications of IMU acceleration and angular rate. Therefore, the centralized navigation filter gives the unique navigation solutions. FIG. 2 describes the architecture of the integration mode.
The integration mode can dramatically improve the accuracy of the integrated navigation system better than as the loosely-coupled mode does. But, the integration mode can not enhance the dynamics of GPS tracking loops and increase anti-interference capability of the GPS receiving set because of lack of the inertial aiding information available to the GPS receiver tracking loops. Its main advantage is that almost all GPS receivers in the market can be conveniently integrated with IMU sensors into a GPS/IMU integrated navigation system, and there are no special requirements for GPS receivers.
C. GPS/IMU xcfx81/xcex94xcexd+xcex94xcex8 Integration Mode with Aiding of GPS Tracking Loops:
Traditionally, this integration mechanization is called the tightly-coupled architecture. The obvious distinctions between the above mode in B and this mode are the levels of information fusion and requirements for the GPS receiver. This mode requires that the GPS receiving set must accept the aiding information from the integrated navigation Kalman filter for the GPS receiver code tracking loop aiding. And the Kalman filter must output velocity and acceleration (V-A) information at a high data rate in order to allow the aiding information to be available in the GPS receiver. FIG. 3 shows the architecture of this integration mode.
The integration architecture more effectively utilizes GPS and IMU measurements and a priori information to determine and correct for system errors in a highly integrated fashion. It also yields better performance than the above two systems, providing more accurate navigation estimates during periods of a high dynamic flight or jamming environment. In the integration mode, the design of a tracking loop mechanism for GPS sensors imposes the requirements on both the hardware and the software algorithms for reception and processing of the velocity and acceleration (V-A) aiding information from the navigation filter. Its main advantages are the enhancement of anti-jamming capability and improvement of adaptability for high dynamic environments of the GPS receiving set. From the view of the system design, the integrated navigation system design faces more challenges in GPS receiver digital signal processing, tracking loop aiding, data exchange and system integration.
D. GPS/IMU xcfx81+xcfx86/xcex94xcexd+xcex94xcex8 Integration Mode without Aiding of GPS Tracking Loops:
The integration mode is similar with the architecture in the above mode B. The difference, however, is only in the type of information fusion as shown in FIG. 4. In the integration mode, the kinematic GPS technique is utilized in order to obtain more accurate GPS measurement data.
The GPS carrier phase measurement can obtain the sub-centimeter measurement accuracy. But, the phase integer cycle ambiguity and cycle slip problems in the carrier phase observable limit the obtainment of highly accurate positioning solutions, especially for on-the-fly phase ambiguity resolution and cycle slip detection in a high dynamic environment. Once the phase ambiguity and cycle slip problems are solved, the integrated system can achieve better positioning accuracy than the above other systems can do. In addition, the integrated system has no special requirements for the GPS receiver except for the carrier phase measurement. Main disadvantage is that the integrated mode can not improve the original dynamic performance of the GPS receiver.
E. GPS/IMU xcfx81+xcfx86/xcex94xcexd+xcex94xcex8 Integration Mode with Aiding of GPS Tracking Loops: We call this integration mode the fully-coupled integration mode. In the fully-coupled xcfx81+xcfx86/xcex94xcexd+xcex94xcex8 integration mode, all available GPS measurements are integrated into a centralized navigation Kalman filter with the IMU measurements. The integrated velocity and acceleration information is also used to aid the GPS receiver""s code and carrier phase tracking loops in order to improve the anti-interference capability and dynamics of the GPS receiving set in a tactical high dynamic flight environment. It is now a challenging problem to develop the kinematic GPS/IMU integrated navigation system with the above integration features.
This integration mode is the primary challenge in the research and development of various integrated navigation systems. The accuracy, reliability, dynamic performance and anti-jamming capability of the integrated navigation system are dramatically enhanced through use of the integration mechanism.
In the past years, some kinematic integration methods used in GPS/IMU navigation systems are effective under a low dynamic environment or limited flight environment, for example, in marine navigation and aerial photographic aircraft. However, many kinematic GPS/INS integration systems with the architecture in D only consider how phase ambiguity and carrier cycle slips are resolved and detected. But they do not further consider how IMU velocity and acceleration information can be utilized for aiding the GPS receiver tracking loops (Delay-Locked Loop or code loop/DLL and Phase-Locked Loop/PLL). Unfortunately, such integration methods do not improve the dynamic tracking capability of the GPS receivers although they increase the accuracy of the integrated navigation system and provide IMU instrument errors compensation during the period of GPS receiver operation. In such integration technique, the IMU only provides the estimated position and velocity for the GPS receiver to reduce the phase ambiguity search space, and ranging errors corrections in order to increase the navigation accuracy. Once the GPS receiver loses its lock-onto satellite signals, there is no further link between the GPS receiving set and the IMU sensors.
Each one of the above integration modes has its advantages in the corresponding performance/cost and synergy efficiency trade-offs, flexibility and simplification of the realization and redundant navigation solutions. But, available observables used in the current integration methods are mainly code pseudorange and range rate in addition to general acceleration and angular rate outputs of IMU sensors. The velocity information derived by IMU sensors is mainly utilized to aid the frequency lock-on of the carrier-frequency tracking loop (Doppler removal before codes matching) for the purpose of code delay measurement but not that of carrier phase measurement. Therefore, the range measurement accuracy is still limited by code tracking loop bandwidth and resolution. Especially, in order to acquire the GPS satellite signals in the high dynamic environment, the bandwidth of the closed loop of the unaided carrier-phase tracking loop in the GPS receiver must be wide enough to adapt to the fast GPS signal frequency and phase changes caused by high dynamic motion. It is fairly difficult to achieve without external aiding tracking information because there exists unwanted interference noise which will simultaneously enter the tracking loop with a wider closed-loop bandwidth.
Moreover, a fully-coupled kinematic GPS/IMU algorithm (FCKGA) navigation software package is incorporated the present invention which efficiently utilizes the developed results in this invention, such as robust centralized Kalman filter, IMU-aided on-the-fly widelane ambiguity resolution and IMU V-A aiding tracking loops.
The successful development of the FCKGA navigation software system will give e a competitive edge with sophisticated navigation and guidance systems. This advanced system is featured with the following important advantages:
(1) Hardware-Level Redundancy: In the fully-coupled integration mode, the GPS receiving set is used as one of the sensors (GPS, gyro and accelerometer) of the integrated navigation system. The restriction of at least 4 satellites for navigation solution computation can be significantly relaxed. The hardware-level redundancy will help to enhance the reliability of the overall system through fault-tolerant software design.
(2) Use of Low-Cost IMU Sensors: In the FCKGA-based system, precise positioning results can be used to routinely correct IMU instrument errors in flight. Therefore, low-cost non-INS-grade inertial sensors can be utilized in the integrated navigation system.
(3) Minimum of Tracking Loop Bandwidth and High Anti-Interference: In the fully-coupled integration mode, the V-A solution of the integration navigation filter is transferred as V-A information along the line-of-sight between the GPS satellites and the integrated system, and fed back to the digital signal processor of the GPS receiving set at a high rate. In the signal processor, the V-A information is used to compensate the high dynamics. Therefore, the fixed bandwidth of the tracking loop can be reduced to a minimum to prevent unwanted interference noise.
(4) Fast Phase Ambiguity Resolution/Cycle Slip Detection On-The-Fly: The precise positioning results of the integrated system can generate the computed range between satellites and the navigation system. The computed range is compared with the measured range between the satellites and the navigation system, and the resultant difference can be utilized to detect cycle slip and reduce the ambiguity search space efficiently.
(5) High Navigation Accuracy: The integrated navigation system uses the kinematic GPS technique with centimeter-level measurement accuracy to significantly improve the navigation accuracy of the integrated system. Once atmospheric delays and selective availability (using dual-frequency and an authorized GPS receiving set), phase ambiguity and cycle slip problems (using methods developed in this invention) are solved, the navigation accuracy of the integrated system only depends on the IMU instrument errors. The integrated navigation system is designed to perform IMU dynamic correction and alignment. Furthermore, the integrated navigation output is at the rate of the INS output.